Improve encoding

[mikowitz]

Make improvements to image encoding:

• Parallelize converting rows of pixels to binaries
• Better calculation of necessary bit depth and color mode for optimized file size. In order, checks if the image can be encoded using:
  • grayscale, bit depth 1 ( black and white)
  • grayscale/alpha, bit depth 8
  • indexed, bit depth 1,2,4,8 depending on how many colors in the palette
  • truecolor/alpha, bit depth 8
• Allow encoding options to be passed to ExPng.Image.to_file/3
  • right now it only allows compression level

Adresses #11